Synthetic polymers are frequently combined with glass fibers to obtain a composite product whose properties, particularly the mechanical properties, are better than those available with the use of unreinforced polymer material. This is particularly true in the case of polyolefins.
When reinforcing polyolefins with glass fibers, it is known to treat the surface of the fibers with a finish comprising a polyolefin emulsion in the aqueous phase. This finish, in addition to protecting the surface of the glass fibers, which is one of the standard functions of this treatment, improves the adherence of the fibers to the material which they are to reinforce. To optimize the compatibility between the finish covering the fibers and the matrix to be reinforced, it also is known to use an emulsion having a polyolefin base of a nature close to that of the polyolefins constituting the matrix.
Emulsifying polyolefins is relatively easy as long as their weight average molecular weight is less than about 10,000. The polyolefin is simply melted while adding suitable emulsifying agents with stirring. An emulsion is then obtained by subsequently adding the necessary water. U.S. Pat. No. 3,655,353 to Nalley et al. describes an emulsion with a polypropylene base formed according to the process described above.
It is not as simple, however, to emulsify polyelfins when their weight average molecular weight is greater than about 10,000 since, in the molten state, the fluidity of such polyolefins quickly becomes insufficient to obtain an emulsion. Moreover, isotactic or syndiotactic polyolefins, which exhibit a strong tendency to crystallize during their cooling, are also difficult to emulsify. Because of this crystallization tendency, the polyolefins generally selected to be emulsified are atactic or amorphous polyolefins.
Several patents recommend techniques for reducing the problem of crystallization. For example, U.S. Pat. No. 4,240,944 to Temple discloses the emulsification of a mixture of an amorphous polypropylene and an isotactic polypropylene after first melting the mixture of these two polymers. The proportion of isotactic polypropylene in this mixture is less than or equal to 50%.
To obtain emulsions from isotactic polyolefins of high molecular weight, it also is known to dissolve them with heat in an organic solvent which is immiscible in water, with subsequent addition of the necessary water. Such a process is described, for example, in French patent No. 2 588 263. This process requires, however, the subsequent elimination of the solvent by extraction or by washing and drying.